Is it possible to make stains from a fluid simulation?
Making stains from fluid stimulation would hamper your scene performance as it would cost too many polygons (Fluids are expensive) and also it will be really difficult to do something like that as fluids are meant to flow not stay on a mesh like stains so the simplest and most effective way is to use texture projection to project stains on your mesh or you can also make custom texture combined with some roughness and bump maps to make some part of your texture look like stain.